The present invention relates to the field of nuclear spin tomography, and in particular to a shield for delimiting an electric field in a high-frequency apparatus for nuclear spin tomography. An apparatus of a type to which the present invention relates includes a high-frequency antenna for exciting nuclear spin in a biological body to be examined and for receiving high-frequency signals produced by the nuclear spin excitation. Such systems often include a shielding means arranged between the antenna and the body to delimit an electric field coupling. An exemplary system is disclosed in European Patent A-0 047 065.
As is known in the art, nuclear magnetic resonances may be used to produce cross-sectional images of a biological object to be examined, such as a human body or part of a body. The body to be examined is brought into a strong, homogeneous magnetic field, called the base field, which produces within the body an alignment of nuclear spins of atomic nuclei, and particularly of hydrogen atom nuclei (that is, protons) bound to water. These nuclei are then excited by high-frequency excitation pulses into a precessional movement. At the end of each exciting pulse, the atomic nuclei precess with a frequency which is dependent on the intensity of the base field. After a predetermined period of relaxation, and based on their spin, the., nuclei move in pendulum fashion back into a preferred direction established by the base field. Using computer and/or measurement analysis of the integral, high-frequency nuclear signals, an image can be produced from the three-dimensional spin density or from the distribution of the relaxation times within a layer of the body. Linear field gradients are used to associate the nuclear resonance signals detectable as a result of the precessional movements with the specific site of their origin. For this purpose, the corresponding gradient fields are superimposed on the base field and controlled so that excitation of the nuclei takes place only in the layer to be imaged. Imaging based on these physical effects is known as nuclear spin tomography (NST) or nuclear magnetic resonance (NMR) tomography.
Tomography systems generally require a transmitter with a corresponding antenna to generate the high-frequency excitation of the nuclear spin. As disclosed in Wilhelm Durr et al., High Frequency System for Nuclear Spin Tomography, ntz Archiv, Vol. 11, No. 5 at 237-243 (1989) and European Patent B-0 073 375, the antenna can be implemented as a whole-body resonator. For this purpose, the transmitting antenna is developed as a resonant circular (hollow) waveguide antenna. It therefore has several electric conductor elements extending parallel to the cylinder axis of the base-field magnet, which is developed as a solenoid. These conductor elements are surrounded by a common envelope tube, or HF-screen, made from a material exhibiting good electrical conductivity, and which is pervious to the low-frequency gradient fields but impervious to the high-frequency field. Resonant oscillation conditions are established for the transmission within this circular waveguide antenna.
Where regions of relatively limited extent are to be imaged, a surface coil or local coil may be placed on the part of the body being examined, such as a vertebra, the middle ear or an eye. In a simplest case, such a local coil consists of a circular wire loop with at least one separation bridged by a capacitor and connected in high-frequency. In general, such a local coil is used only to receive the high-frequency signals produced by the nuclear-spin excitation. The nuclear spin itself is generated by a transmitting antenna; for example, a whole-body antenna designed as a circular waveguide antenna. In one suitable embodiment of a high-frequency system, two different high-frequency antennas are used for producing an image; namely, a high-frequency transmitting antenna and a high-frequency receiving antenna. The transmission and reception functions may alternatively be provided by a single antenna, as disclosed in European Patent B-0 073 375. A suitable antenna arrangement is also disclosed in the co-pending United States patent application entitled "High-Frequency Apparatus for Nuclear Spin Tomography," assigned to the same assignees as the present invention, the disclosure of which is incorporated herein by reference.
Antenna arrangements of known high-frequency systems for nuclear spin tomography successfully achieve a desired coupling between the biological tissue of a body to be examined and the antenna arrangement through a magnetic field (the B-field); however, these arrangements also exhibit an undesired coupling through an electric field (the E-field). This electric field coupling can lead to undesired heating of the surface of the biological tissue during transmission, particularly on the surface of the skin. During reception, the electric field coupling produces additional disturbing therma in the form of noises in the high-frequency signals produced by nuclear spin excitation, thereby worsening the signal-to-noise ratio of the high-frequency signals. It is thus desirable to minimize electric field coupling as much as possible, limiting it to a bearable amount.
To this end, the aforementioned European Patent A-0 047 065 discloses a shielding means, known as a Faraday shield, disposed between the antenna arrangement and the body to be examined. The shield consists of a wire frame of electrically conductive material, such as copper, formed from a plurality of annular wire loops spaced apart from each other on the outside of a support tube which surrounds the body to be examined, with the loops electrically connected to one another by pieces of copper. The shield is thus a quasi-mesh-like construction having a predetermined mesh width. In this arrangement, however, the electrically conductive wires of the wire frame must be spaced relatively far from each other to avoid influencing the B-field. As a result, suppression of electric field coupling is poorly pronounced.